The OP is asking about headphones, so let's try to stay on that topic.
In my experience, the headphone portion of Heil headsets are stereo. The plugs are TRS (aka stereo) and the speakers are wired separately.
The last monaural headphones I purchased were from Radio Shack and were specifically marketed towards scanner and shortwave listeners.
Unfortunately, there isn't much market for monaural headphones these days. There may be some specialized, and expensive, products on the market which meet the OP's quest. Or, just find a pair of stereo headphones that you like and wire the two sides in parallel (unless that would make the impedance too low for your radio).